Electrical prospecting



Jan. 19, 1932. c. R. NICHOLS ET AL I 1,842,362

ELECTRICAL PROSPECTING Original Filed July 19. 1927 z's t s 1 INVENTORS n CHI/PA 5.5 /P. lV/C/YOLS 5/44 1054 W/u /.5 To

Q 542 d' 'ATTORNEY Jan, 19, 1932.

c. R. NICHOLS ETAL ELECTRICAL PROSPECTING Original Filed July 19. 1927 2 ,Sheets-Shet 2 ATTORNEY Patented Jan. 19', 1932 UNITED STATES PATENT OFFICE,-

cnmns a, NICHOLS m) smear. a. WILLISTON, or mines, mas

innermost rnosrnorme' Application filed July 19, 1927, Serial No. 206,829. Renewed November 12, .1981.

The general object of the present invention is to provide an improved method of, and apparatus for determining thelocation and character of the sub-surface-bodies or earth liportions of diiferent electrical resistance -from the adjacent earth portions. Our present invention was primarily devised for use in locatingdeposits of petroleum, but the invention 'is well adapted also, to the location 10 of ore deposits of higher conductivity than sistance of thosebodieswere substantially lower than that of the adjacent earth. We

20 recognize the possibility that such displacements may be due to saline matter associated with the oil sands and not to the direct action the case. i

In the use of the present invention we energize an observation field by causing an electriccurrent flow through the earth between points'thereof on which. different potentials b areimpressed, as by means of electrodes inserted in the earth and connected to the terminals of an-electric generator or generators,

. in order that the resultant jcurrent flow. distribution through the earth may be determ exploration field for mined, or compared-with the current distribution which would result from such ener- I gization if the earth beneath the observation field were of uniform conductivity.

While others have (roposed toenergize an the purpose. mentioned,

our invention is distinguished from all prior I methods of electrical prospecting known to us in the manner in which the observation field is energized, and in particular by the fact that our mefhodof energization results in the creation of equi-potenti'al lines which intersect'at a point in the earths surface and thereby define a potential center, whereas the methods of energization heretofore practiced or proposed do not create intersecting equipotentiallines defining potential centers, or points, as distinguished from lines.

The potential centers created by the practice of our invention are comparatively easy to locate, and their location ordinarily gives more valuable information'than can be ob tained by tracing out the non-intersecting equi-potential lines producedby prior methods of energization, even though amuch larger number of points are located'bn said-nonintersecting lines, than are required for the location of potential centers produced by the practice'of the present invention.

In a simple mode of practicing the present invention the energized points are located at the corners of a simple geometrical figure so that a potential center will be formed in the field 10f obseryation by the intersection of iso-potential linesipa'ssing between different pairs of energized points. The potential center thus formed will coincide withthe geographical center of said geometrical figv h l tt but are not certain that this is ure if the earth resistance beneath the exploration field is uniform,'but if oil bearing sands or other disturbing bodies ,lie beneath be produced and located. For example, with energization points located at the corners of a square or rectangle, it is possible, by succes-;

sively varying the relative strengths of the currents flowing through the energization points of the same polarity, tosuccessively create a-large numberof diflerent potential centers without change in the location of the points of energization. The exact point on the observation'field at-which each such potential-center will be located if the earth resistance beneath the exploration field is uniform, can be predetermined, and the displacement, if any, of the observed potentialcenter from said point gives readily interpreted information concerning the location and character of disturbing bodies beneath detail.

the surface of the exploration field. In addition to the two specific modes of practicing the invention already mentioned, there are others which may be employed, some of which are hereinafter referred to in some employed in.radio receiving circuits.

In general, to cover any extended field of exploration, it is necessary with our improved method, as with the methods of our prior applications hereinafter mentioned, to successively establish a plurality of different observation fields each covering a difi'erent portion of the exploration field, and to determine the character of the earth structure below the exploration field from the information collectively furnished by the. observations made in connection with the different observation fields.

The potential center displacement method of electrical prospecting permits of the rapid systematic and relatively inexpensive exploration of an extended field of exploration,

. and is characterized by the ease and comparative accuracy with which the" observation results obtained may be coordinated, compared and interpreted. With the present invention the observational data obtained are always direct and positive, in that the direction of potential center displacement is a definite indication of the relative locationsof the geographic norm of each obvention.

servation field and the disturbing body. The results obtained are thus more direct, and positive th an' those obtained with methods of electrical prospecting heretofore proposed in which the effect of a disturbing body on the elongated iso-potential lines to be located,

Of the drawings: Fig. 1 is a chart diagrammatically illustrating current flow. distribution and potential obtained by an unequal distribution of current among different energization points;

Fig.5 is a diagram illustrating diflerent potential center locations obtained by varying current distribution;

Fig. 6 is a diagrammatic representation of an alternative method of energizing an observation field; n.

Fig. 7 illustrates, another method of creating a potential center in an observation field; and

Fig. 8 is a'map of a portion of an observation field showing .iso-potential lines traced in locating a. potential center displaced from the geogra hie center of the field.

1 In the o servation field energizing system diagrammatically illustrated in Fig. 2, the energization points A, B, C and D are located at the corners of a square, the diagonally opposed points A and C being connected by correspondin conductors F to the posit1ve side of a ireet current generator G,

' while the other two energization oints D and B are connected by correspon ing conductors F to the negative 'side of the generator. Variable resistances R, R, R and R are shown as provided in the conductors F running to the different points A, B, C and D. By adjusting the resistances, for example the resistances R and R in two-circuit branches F, of opposite polarity, the same current fldw into or' out of the earth at each energization point maybe secured. EA, EB, EC and ED are ammeters in the different conductors F the readings'ofwhieh indicate when and 7 how the resistances R and R need to be adjusted to obtain and maintain the desired uniformity of energization. Contact between the corresponding conductor F and the earth at each of the energization points A, B, C and D may be effected in any usual or suitable manner. Ordinarily this contact is effected by inserting a lurality of metallic pegs or electrodes 1n tE eter, the center of which may be assumed to be, the energization point, mathematically considered, especially as the area in which the yards, up to 2 or 3 or more miles. To insure 'good contact, and to avoid polarization difiiculties, the ground in the vielnity of each elec-. trode eglmay be wetted with a solution-of a salt 0 t e metal forming the peg, for ine earth'distributedover an area of 10 to 100 feet or so in diamstance, a'solution of copper sulphate when the pegs are made of brass, or may be Wetted by a solution of any other salt adapted to serve as an electrolyte, or the earth may simply be Wetted withwater in many cases.

. Fig. 1 is a'chart illustrating the direction of current flow and potential variations at the surface of the earth forming an observation field energized as shown in Fig. 2 when the portion of the earth near enough to the ob 'servation field to significantly affect the flow distribution and potential variations at the surface of the field of observation, is of uniform conductivity. In such case there will be an iso-potential line K passing midway between the energization pointsD and C, and

midway between the energization points A and B, and a second iso-potential line L passing midway between the points A and D, and V 36 equi-potential line. The iso-potential lines K and L may also be called zero potential lines since no changein potential at points along those lines is created by the artificial earth-,energization current flow. In addition to the zero-potential lines K and L,

any number of other iso-potential lines which are not zero 1potential lines, may be traced on the field of o servation. 'Some'of these other iso-potential lines are shown in Fig. 1, and are designated by the symbol I. The iso-potential lines are closed curves. If the-earth were of uniform conductivity, the lines L and K would theoretically be great circles, and each other iso-potential line I would lie in one of the quadrants formed by the intersection of the planes including the great circles K and L. In practice, of course, the distance from an observation field atwhich measurable potential differences are created by the energization of the corresponding points A, B, C and Din ordinary electrical prospecting work is nothing like as great as the half circumference of the earth. The direction of current flow at; the earth surface or inthe earth at any point is normal to the iso-potential line or surface at that point. In Fig.

'- 1, the'lines H indicate-the manner in which the direction of current flow varies over the observation field. 1

In locating the potential center Mof an observation field energized in accordance with the present invention, the general method of procedure is to shift exploring contacts neighborhood of the geographic center we of the latter or in the neighborhood of the expected position of the potential center M,

until a galvanometer or other detectingim strument EE connected to the exploring con-,

tacts shows the latter to beat the same potential when either is moved along a line in-- tersecting a line on which the other exploring control is located. This condition can on y exist whenthose lines are intersecting equi-potential lines, as are the lines K and L of Fig. 1. In general the detecting circuit including the contacts 0 and N and instru-- ment'E may be identical with arrangements heretofore proposed for locating points on iso-potential lines. The potential center M may be located in actual practice, without direct location of either intersecting line K and L. After locating a few. ppints on the curves I shown in two opposing quadrants in Fig. 3, it is possible to draw lines between correspondingly located pointson the opposite limbs of the curves I, which intersect at the potential center M or as near thereto as is practically required.

In the map shown in 8 the circles 11.

represent exploring contact position points on iso-potent1al lines I, K and L actually located in accordance with the procedure described in Fig. 3 in determining the position of the potential center M of the field of observation in which the potential center is displaced from the geographic center m of the field by sub-surface oil bearing sands. -In

Fig.- 8 more exploring electrode position points are shown than are ordinarily required in a single observation field to determine the displacement of-the potential center by oil bearing sands beneath the field.

In practice the detecting circuit illustrated in Fig. 3 may well include such amplifying provisions and provisions for eliminating the stray earth currents as are disclosed and claimed in our prior application Serial No. 129,430, filed August 16, 1926. In practice,

also, the energization currents supplied by the generator G may advantageously be interrupted and reversed at regular intervals as described in said prior application. The features of our invention disclosed in our prior a plication and referred to above need not be escribed in detailherein, however, as

they form no part of the present invention,

and while they may advantageously be used in carrying out the present invention, the latter may be carried out in other ways and by I other instrumentalities.

Fig. 4 illustrates the location of the potential center M formed with the apparatus shown in Fig. Z when the various resistances R, R R and R are so adjusted "that the same current flow between the earth and the generator is had at the energization points D and B, while the current flow between the .O. and N over the observation field in thee earthand the generator is greater 'at the point unequally energized points A andC, are equiat and B; and M is the potential center.

potential lines IA and IC, respectively, com prising clearly defined opposing corner portions, the apices of which coincide at the p0.- tential center M.

The potential center M thus formed b the intersection of the equi-potential lines and-IO can be located with comparative ease.

-When the resistance of the earth beneath the exploration field is uniform, the center will lie on the line AC, and at relative distances from the points A and C which depend ;u on the, division of current passing between t e gen erator and the earth at the points A and C, and can be accurately calculated by a'proper application of Ohms law. A disturbing body beneath the earths surface may shift the point of potential center M along the line AC, or laterally to one side or the other of the latter. In either event the displacement of the actually located potential center from the point at which the center would lie. if the earth underlying the exploration field were of uniform resistance maybe utilized directly or indirectly to determine the character and location of the disturbing bod y or bodies creating the displacement.

By following the general procedure described in connection with Fig. 4, 'it is pos sible to successively create and locate a multiplicity of potential centers at difi'erent points in the surface of an observation field energized at one and the same set of points A, B, G and D. For example, in Fig. 5, M "--'--M represent different potential cen-' ters which may be located on the surface of an observation field overl ing a portion of the earth of uniform con uctivity by a few simple changes in the relative strength-of the currents flowing between the source of If the current flow at A is twice the current flow at C the point M will not be as near C as it will if the current flowat A is three times that at C. By thus changing the relative current strengths at the difi'erent energization, points, other sets of potential centers like the set shown 'in Fig. 5, may be located in the same observation field without changing the location of the energization points A, B as locatin bodies of oil bearing sands it Willbe or inarily unnecessary to create and utilize'a great multiplicityof potential cen ters in a single observation field, and in many cases the location of a single potential center may well be suflicient if the potential center so located is not displaced as a result of the presence of a disturbing body beneath the" field. In determining the character of a non-- homogenous earth structure beneath a large observation field, however, itmay well be advantageous to successively create a considerable number of potential centers, and in particular to create more than one such set of points as are shown in Fig. 5, incertain, at least, of the difi'erent sections oithe field of observation. By suitably varying the current distribution between the different energization points, it is p'ossible'to create a potential center within each of'a number of comparatively small sized units or sections into which the observationfield may be divided.

Itjwill be'readily apparent that a greater number than two pairs of equally energized energizati'on points, equally spaced along a circle and alternating inlpolarlty, will result current and the earth at the energization circle when the earth resistance below the obpoints A, B, C and D. In Fig. 5, M represents the same potential center as is shown in Fig. 4. M represents the potential center created by decreasing the current flow at the point C and increasing it at the point A until the last mentioned current fiow exceeds the current flow at C in a predetermined amount. The potential center M is created by equal current flows at A and C and a greater current flow at D than at B. M represents the po-' tential center created when the current flow at A and C is the same and the current flow at B is greater than at D.

M represents a potential center created when the current flows at C and B are alike and are less than theequal current flows at A and D. M? isa potential center defined by making the current flow at A equal to that at B and less than each of the similar current flows at C and'D. M is a potential center created by making the current flow at D and 0 equal and less than the similar current flows at B and D.

servation field isuniform. The same general result may be obtained if the pairs of energizationpoints A and B, C and D, etc. are not I e ually spaced, but are arranged as shown in Flg. 6, so that'two points of each pair are spaced relatively closelytogether, whilejadjacent pairs of points are more widely spaced apart. The arrangement shownin Fig. 6, is o especial'utility where in order to avoid laying transmission lines, or for other reasons the two energized points of each pair are energized by an individual generator or source of current shown as a separate generator GA; R is a. variable resistance, and E is a meter placed in each energization circuit shown in Fig. 6. With the arrangement shown in Fig. 6, the energized points of each pair need be separated but a few hundred feet, even though the. field of observation may be so large that the distance between'any one pair of en ized points and any other pair. of energiz ii points amounts to thousands of feet. As shown in Fig. 6, the energization oints v A, B, C, D, a, b, c and d are spaced a out a senses and between the pointsa and b, and be-- tween the points sand 01, which is relatively small as compared with. the distance between the points B and C, and the similar distances between the oints D and a, and between the points 12 and a, and between the points at and A. With. the arrangement shown in Fig. 6, there is a mid-potential line K passing between the points A and B, andbetween' the points I; and a, and a second zero potential line K passing between the points C and D',

and the points (1' and c; and in addition a zero potential line K passing between the points B and C and the points a and b, and a zero potential line K passing between the points I) and a and between the points A and d. These zero potential lines all intersect at the potential center M which corresponds with the geographical center of the circle on which the energization points are located, when the earth resistance beneath the observation field is'uniform. It will be readil apparent that intersecting zero potential nes may be formed in the manner shown in Fig. 6 with three, or any larger number of suita ly located pairs of energization oints.

In practice we regard'four pairs 0 such points as preferable under ordinary conditions of use, to three, or five or more palrs.

Fig. 7 illustrates howa potential center M" may be created within an observation field energized by a plurality of three or more energization points of'thesame polarity located at points spaced about the potential center and operating in conjunction with envergization points of opposing polarity suitably located without the field of observation. 4

As shown in Fig. 7, there are four energization points A, B, C, D, similarly energized and of like polarity located at points equally spaced about a geographical center point. As

shown, the point A is energized by a current source G having one terminal connected to the earth at A and the other connected to the earth at a point. A more remote from said eogra hical center point and on the line min t e latter gassingfihrough the oint A. The points B', Y and are similar y energized by other current sources G in conjuncment illustrated in Fig. 7, similar closed equipotential lines 1 1 ,1 etc. may be traced v on the earths surface about the potential center lil? which is enclosed by each of said lines.

. Each such equi-potential line 1 1 and I",

with the. arrangement of energization points shown in Fig.7, is a figure having four sides each of which is concave. When the earth and a Vertical axis, and havlng its apex in the earths surface at the potential center point M The different concave sides of said pyramid correspond with portions of bowl-shaped 'equi-potential surfaces in the earth curving aboutthe different energization points A, B, C and D. The corners of the pyramid represent lines formed by the intersection of the said bowl-shaped surfaces about adjacent energization points, and these lines of intersection all intersect at the apex of the pyram1 With the points A, B, C and D of Fig. 7 located at uniformly spaced points on a circle, and with the same current flow at each point, the potential center will lie at the center of the circle, provided the earth resistance beneath the observation field is uniform. However,

by var ing the relative current strengths at the di erent observation points different p0- employed may be varied. In general, how- 7 ever, practical considerations and ease of interpreting the observation data obtainable ordinarily require the use of a comparatively small number of energization points and their symmetrical disposal at the corners of symmetrical geometrical figures. While the potential center M created with the arrangement shown in Fig. 7, is at the intersection of equi-potential lines, those lines do not lie on the earths surface, and the potential center cannot be located by tracing out equi-potential lines on the earths center which intersect at the potential center.

The potential center M of Fig. 7 is like the potential centers created with the arrangements shown in otherFigs. however, in'that it is apoint on the earths surface on which an identifying potential characteristic is impressed by the energization of the-observation field. If the portion of the earth beneath the observation field is not of uniform resistance, the potential center created with each form of the invention illustrated herein is displaced from a predetermined geographic point in a direction and to an extent which can be measured, and which gives information as to the character and-location of the 95 tential centers maybe successively determined disturbing bodies of greater value and more readily obtainable than the information ob- .tainable with any prior. method of electrical;

prospecting known to us and practicall available under conditions of use for whic the present invention is especially well ada ted. The specific form of the invention disc osed and enericall claimed herein and shown articu arly by igs. 1, 2, 4 and 6 in which t e potential center located is formed by the intersection of zero potential lines, is disclosed but not claimed in our prior application Serial No. 147,310, filed November 9, 1926, of which this application is a continuation in part. Certain generic features of invention disclosed herein andin our copending application'Serial No. 303,542, filed September 1, 1928, are enerically claimed in the last mentioned app ication.

While in accordance with the provisions of the statutes, we have illustrated and'described the best form of embodiment ofour invention now known to us, it will be apparent to those skilled in the art that changes may be made in the form of the appartus disclosed without departing fromthe spirit of our invention as set forth in the appended claims and that in some cases certain features of our invention may be used to advantage without a corresponding use of other features.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

1. In electrical prospecting the method of establishing an artificial ground field which consists in creating a potential center point on the earths surface by causing measured electric currents to flow through the earth between separated earth points and there- 1 by creatin .equi-potential lines which intersect. at sai potential center polnt.

2. In electrical prospecting the method of establishing an artificial ground field which consists in creating a potential center point on the earths surface bycausing electric currents of regulated relative strengths to flow through the earth between separated earth points and thereby creating equi-potential lines which intersect at said potential center oint. p 3. The method which consists in establishing an artificial ground field by causing electric currents of regulated relative strengths to flow through the earth between separated earth points and thereby creating equi-potential lines intersecting at a potential center point on the earths surface, and determining the subsurface composition of the earth from the relation of the last mentioned point to a point fiired by its geographical'relation to said earth points.

4. In the electrical'determination of the sub-surface composition 'of the earth, the method of establishing Tfartificial j ground field which consists in energizing points of the earth so displaced and with such opposing polarities as to thereby create two or more iso-potential lines" intersecting at a placed in a predetermined direction from a point centrally located with respect to the rst mentioned points.

6. In the electrical determination of the sub-surface composition of the earth, the method of establishing artificial ground fields which consists in energizing points of the earth so displaced and with such opposing polarities as to thereby create two or more iso-potential lines intersecting at a point and successively'varying the relative de ees of energization of the first mentioned pomts to thereby successively produce differently located iso-potential line intersection oints.

7. n the electrical determination of the sub-surface composition of the earth, the

method of establishing an artificial ound field which consists in simultaneous y and proportionally energizing three or more pa rs of points, the two points of each pair e1ng symmetrically disposed at opposite sides of a line intersecting other lines correspondingly located with respect to the other pa rs of points and the two points of each pair being closer to one another than to points of otherv pairs.

8. In the electrical determination of the I sub-surface composition of the earth, the method which consists in separately energizmg a plurality of observation fields suitably dispersed over the exploration field and in such manner that the energization of each field does not significantly affect the said energization of any other of said fields and so as to form in each observation field a plurality of iso-potential lines intersecting at a point, and comparing the direction and magnitude of dis acement of the potential centers of said o servation fields resulting from lack of conductive uniformity in the portion of the earth adjacent the different observation fields.

9. Inthe electrical determination of the subsurface com osition of theearth, the method step w 'ch consistsnimenergizing points of the earth so disposed and with such op osing polarities as to thereby create definite yproportionedonm fl s through the earth toward or from the difierent points of energization and to thereby create two or more equispotential lines intersecting at a. a point. p '10. In the electrical determination of the v subsurface composition of the earth, the method step which consists in energizing points of the earth so disposed and with such polarities as to thereby create a plurality of equi-potential lines each of which intersects with two other of said lines and which collectively outline a polygonal figure onethe earths surface. v a

11. In the electrical determination of the subsurface composition of the earth, the method of energization which consists in impressing one polarity on each of a plurality of ener ization points located at the corners of a p0 ygonal figure on the earths surface and impressing an opposing polarity on other points on the earths surface lying outside of and distributed about said figure.

Signed at Dallas, in the county of Dallas, and State of Texas, this 12th day of July,

CHARLES R. NICHOLS. SAMUEL H. WILLISTON. 

